Many types of commercial and industrial objects require heat treatment when they are manufactured. For example, certain objects require surface hardening. Surface hardening is achieved by heating the object and then quickly reducing or quenching its temperature. Other objects must be annealed, which is achieved by heating the object and then allowing its temperature to diminish slowly. Coatings on certain objects are heat-cured by heating the object after the coating has been applied. Seamed tubing is created by welding the adjoining edges of a ribbon of sheet metal that has been rolled into a circular configuration. Separate objects are often soldered or brazed together to form a singular part. Heating is also required when forging or forming a product. In these situations, the heat may be delivered from a flame or an electrical source.
Another technique for applying heat is induction heating. Induction heating involves applying an alternating magnetic field to the object. The alternating magnetic field induces eddy currents flowing in the object, and the induced eddy currents heat the object as a result of resistive current flow losses (I2R) in the object. Of course, the effectiveness of induction heating depends on the ability of the object to conduct eddy currents.
The induced eddy currents flow in a circular pattern orthogonal to the direction of the flux lines of the magnetic field. Consequently, the shape of the object has a significant influence on the flow characteristics of the eddy currents, and as a consequence, on the heating characteristics of the object. For example, placing a flat and relatively thin sheet of material in a magnetic field with the flux intersecting the sheet orthogonally is very effective in generating heat in the flat sheet because the induced eddy currents flow in circular patterns within the flat sheet. The invention described in U.S. patent application Ser. No. 11/126,714 makes advantageous use of this orthogonal relationship of the eddy current and the magnetic flux in heating two or more sheets of material to heat-cure an adhesive material between the sheets and thereby bond the sheets together.
Uniform heating is usually required for heat treatment, because the desired characteristics of the heat treatment are generally not achievable unless the heating level is controlled. Uniformity of heating is generally more easily obtained when thermal energy is directly applied to the object, because the location of the flame heaters can generally be established to uniformly heat the object. The uniformity of heat treatment obtained from induction heating can be substantially more difficult, depending upon the shape of the object which is to be heated. For example, uniformly heating the exterior surface of an elongated or nonuniform object with induction heating can be virtually impossible if the surface of the object does not have a geometry which will interact in a uniform manner with the flux lines of the alternating magnetic field. Attempting to use parallel lines of magnetic flux at the exterior surface of a circular object would result in substantially different heating at different locations depending upon the geometry of the curved surface and the spacing of different points on the curved surface relative to the magnetic flux lines.
Induction heating has been used successfully to heat elongated objects by positioning an electromagnetic coil entirely around the elongated object. An example of such a prior art induction heating system 20 is shown in FIG. 1, where a hollow center induction coil 22 surrounds an elongated object represented by a metal rod 24. The coil 22 is formed by one or more windings or turns of an electrical conductor 26, and the metal rod 24 extends through an open center of the coil 22 with the rod 24 circumscribed by the windings of the electrical conductor 26. Electrical drive current 28 flows in the conductor 26 and creates magnetic flux within the open center of the coil 22. The flux lines of the magnetic flux in the coil 22 are oriented generally parallel to an axis through the coil 22 as represented at 30 in FIG. 2.
The magnetic flux 30 interacts with and flows axially through the metal rod 24. The axial flow of magnetic flux in the rod 24 induces an electrical eddy current 32 which flows circumferentially around the rod 24, in accordance with known electromagnetic principles. Each alternation of the phase of the driving current 28 conducted through the coil 22 causes the flow of the induced current 32 to change direction. However, the flow of the induced current 32 is always in the circumferential direction around the rod 24. The frequency at which the induced current 32 changes direction is directly related to the frequency of the magnetic flux 30, and the frequency of the magnetic flux 30 is established by the frequency of the drive current 28 conducted through the coil 22. Higher frequencies tend to cause the current 32 to pass through the outer surface or skin of the rod 24, while lower frequencies tend to induce the current 32 to pass more interiorly within the rod 24. In either event, the induced current 32 heats the bar 24, as a result of the induced current flowing through the inherent resistance of the conductive material of the bar and dissipating electrical power as heat. As a result, the temperature of the bar increases.
Because the induced electrical current 32 passes completely circumferentially around and through the bar 24, the bar is uniformly heated around its circumference. Assuming that the characteristic resistance of the bar material is uniform, the induced current passes through the entire circumferential path of uniform resistance material, thereby creating uniform heating along the circumferential path in which the eddy current 32 flows. The amount of heating along a linear portion of the bar is controlled by the speed at which the bar 24 is moved through the coil 22. Moving the bar more slowly creates more heat in the linear portion, and moving the bar more rapidly creates less heat in the linear portion, under conditions of a constant amount of magnetic flux 30. By moving the bar at a uniform axial rate through the coil 22, the bar is uniformly and completely heated along its length.
Using the induction heating coil 22 with an open center through which an elongated object is inserted and moved in a controlled manner allows the entire length of the elongated object to be uniformly heat treated with an induction heating system 20 (FIG. 1). Thus, an elongated object, such as an axle, can be surface hardened by heating the surface of the axle in the coil 22 and then quenching its temperature. An elongated object, such as metal wire, can be annealed by heating the wire in the coil 22 and thereafter allowing the temperature to dissipate slowly. Welded seam tubing may be created by extending a circular rolled metallic sheet through the coil 22, and inducing enough circumferential current in the circular rolled sheet to arc-weld the abutting edges together.
The use of an open center induction heating coil creates certain difficulties. It is impossible to remove the induction heating coil from around a lengthy elongated object without cutting the object to withdraw the object from within the coil. While cutting the object may be acceptable in certain circumstances, or not a factor in circumstances were the object itself is relatively short, cutting the object creates significant problems in very long objects, such as welded seam tubing or long wires or cables. Under normal circumstances, it is not necessary to remove the open center induction heating coil from around the elongated object, but in those circumstances where some manufacturing difficulty has occurred, it may be necessary to remove the elongated object from the coil.
Another difficulty in using an open center induction heating coil is that not all types of elongated objects will fit through the heating coil. Only those objects which have a uniform diameter or outside configuration will normally be uniformly heated. Any ridges, extensions or other nonuniformities in the elongated object may prevent it from extending through the coil or prevent adequate heat treatment of the object. If a larger coil is used to accommodate the nonuniformities, the variations in diameter or distance from the center of the object will have a variation on the amount of eddy current induced by the coil, and will therefore create variations in heating along the length of the elongated object. Moreover, it is difficult or impossible to heat treat only a portion of the elongated object without incurring the difficulty of manually inserting or threading the elongated object through the coil to locate the coil around that portion of the elongated object which is to be heated.
A further disadvantage of the open center coil induction heating system 20 (FIG. 1) is its relative inefficiency. The coil 22 is required to conduct enough drive current 28 to elevate the temperature of the rod 24. A typical coil 22 may be required to conduct a drive current in the neighborhood of 3000 amps. Such a high drive current causes the coil itself to heat because of the conduction of the high drive current. The amount of heat produced in the coil 22 itself from conducting the high drive current 28 is sufficient to melt the conductor 26 unless the coil 22 is cooled. It is for this reason that the electrical conductor 26 of the coil 22 is formed from an electrically conductive tubing (not shown). The tubing defines a central passageway for carrying a liquid or gas coolant (not shown) through the tubing of the coil 22 to remove the heat created by the drive current 28 conducted in the wall of the tubing. The tubing which forms the coil 22 must have adequate wall thickness to withstand high densities of the drive current 28. The size of the interior passageway in the tubing must be large enough to carry enough coolant at a sufficient flow rate to remove the heat to prevent destruction of the coil and to maintain a safe operating temperature. These practical considerations limit the minimum size of the coil, and also limit the number of turns or windings of the coil, usually to one turn or a few turns.
A separate cooling system (not shown) is required to circulate the coolant through the tubing of the coil 22 and to dissipate or exchange the heat removed. The cooling system and the coolant must be electrically insulated from the coil 22 to prevent short-circuiting the current 28 away from the coil 22. The separate cooling system 26 increases the procurement cost of the induction heating system 20, as well as its operating cost since energy is consumed in operating the cooling system.
In addition to the separate cooling system, supplying the very high amperage, high-frequency drive current 28 to the coil 22 is also complicated. The typical electrical power 34 for the induction heating system 20 is 480 volt, three-phase, 60 cycle AC commercial power. High voltage electrical power 34 is required to deliver enough electrical power to satisfy the high drive current requirements for the coil 22. A rectifier 36 converts the AC commercial power 34 into DC power. An inverter 38 generates an alternating intermediate waveform 40 having the high-frequency characteristic at which the magnetic flux will be produced by the coil 22. Because the typical inverter 38 is not capable of directly generating the high drive current 28 necessary for the coil 22, the high-frequency intermediate waveform 40 is applied to an intermediate conversion transformer 42. The intermediate conversion transformer 42 converts the intermediate waveform 40 into the high amperage drive current 28 that is then conducted to the coil 22. The drive current 28 has the same high frequency as the inverter 38 establishes for the intermediate waveform 40.
The induction heating system 20 therefore requires four electrical energy conversion steps: a first conversion from 480 volt, three-phase 60 cycle AC commercial power 34 to DC power; a second conversion from DC power to the high-frequency intermediate waveform 40; a third conversion from the intermediate waveform 40 into the very high amperage drive current 28 applied to the coil 22; and a fourth conversion from the drive current 28 to the magnetic field which interacts with the elongated object represented by the metal rod 24. Each of these conversions involves energy losses, because energy losses are simply inherent in the electrical equipment which perform these conversions. As a result, a significant amount of the electrical power 34 delivered to the induction heating system 20 is consumed in these conversions and is therefore lost before the magnetic field has been created. Moreover, a significant amount of energy of the drive current 28 delivered to the coil 22 is consumed in heating the coil 22 itself, and that energy is also lost and is not available to heat the bar 24.
Further still, a significant amount of the magnetic flux generated by the coil 22 is lost by leakage into the air surrounding the coil 22 without interacting with the object. The energy consumed in generating this leakage flux is also lost. The leakage flux results principally from using air as the medium for conducting the flux generated by the coil 22 to the object. Air has a very limited capability of confining and directing magnetic flux, and for that reason, much of the generated flux leaks away before it can be applied to the object. The ability of a medium to confine and direct magnetic flux is referred to as its permeability. Air has a permeability of 1.0, which is the lowest permeability of any material that directs magnetic flux. It is necessary to locate the windings of the coil 22 as close as possible to the object to minimize the flux leakage, but the close location of the windings to the elongated object prevents an object with any significant exterior nonuniformities from fitting through the open center coil 22. As a consequence of the leakage flux, only a reduced portion of the magnetic flux actually generated by the coil is directed into the object and is therefore available to generate the heat necessary to perform work on the object.
All of these losses make coil-type induction heating systems 20 inefficient. Only a small fraction of the energy supplied to a coil-type induction heating system is actually converted into the desired heat. For example, in the neighborhood of only 35 percent of the input energy delivered to a typical coil-type induction heating system 20 is actually available to heat the object. The lost energy is an added cost for operating coil-type induction heating systems 20. Moreover, because of the conversion losses, the rectifier 36, inverter 38 and intermediate transformer 42 must have greater capacities to supply the additional electrical power which will ultimately be lost. The requirement for greater capacity of these electrical components increases the acquisition cost of coil-type induction heating systems 20.